1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to such a method including a step for chemically and mechanically polishing a layer formed on a semiconductor substrate.
2. Description of the Related Art
Recently, a polishing step has become very important in a high integration density device. Heretofore, colloidal silica (particle diameter of 30 nm) stabilized by potasjum hydroxide (KOH) and diluted by pure water to pH of about 9.5 has been used as an abrasive for polishing especially the surface of a silicon oxide film, as disclosed in Journal of Electrochemical Society, Vol. 138, No. 6, pp.1778-1784 (1991). In this prior Specifically, as shown in FIGS. 6A and 6B, a first aluminum-copper (2% copper) wiring layer 602 is formed on a semiconductor substrate 601, followed by forming connection pillars 603 for connecting upper and lower wiring layers are formed on the first wiring layer 602. A silicon oxide film 604 is then formed by the biased sputtering method (FIG. 6A). Next, the silicon oxide film is polished using the abrasive until the surfaces of the connecting pillars 603 are revealed. Finally, a second aluminum-copper wiring 605 is formed, thus completing a two-layer wiring structure (FIG. 6B).
The abrasive, colloidal silica, used in the prior art described above is readily gelled when water is added, and it is difficult to use as it is as an abrasive. 10 Accordingly, potassium hydroxide is added for the purpose of preventing its gelation.
However, the polishing method utilizing the conventional abrasive has the following problems. Namely, the silicon oxide film 604 generally has a high hygroscopicity so that potassium hydroxide contained in the abrasive diffuses into the silicon oxide film 604. Although not shown in FIG. 6, a MOS transistor is formed in the substrate 601, and an alkali metal such as sodium or pottasium infiltrates into the gate oxide film of the MOS type transistor. The threshold voltage thereof thereby varies, which brings about deterioration of its transistor characteristics, and as a result, there is a problem in that deterioration of device characteristics and degradation of reliability are brought about. Moreover, colloidal silica fine particles in the abrasive remain stuck to the semiconductor substrate 601 after polishing, which gives rise to a problem that their removal is a difficult task.
Furthermore, a dried abrasive becomes a generating source of fine particles including those of an alkali metal, which brings about a problem of contaminating the clean room where cleanliness is a prerequisite, and deteriorates the yield of device production and the reliability of the devices.